Carburization resistant high temperature alloy

ABSTRACT

SPECIAL HEAT RESISTANT ALLOYS CONTAINING NICKEL, CHROMIUM, IRON, TITANIUM, ALUMINUM, SILICON AND CARBON AFFORD HIGH TEMPERATURE CARBURIZATION, OXIDATION AND SULFDATION RESISTANCE, CHARACTERISTICS WHICH RENDER THE ALLOYS PARTICULARLY SITABLE FOR USE IN CONTACT WITH SUCH MEDIA AS HYDROCARBONS, HIGH SULFUR CONTENT FEED STOCKS AND THE LIKE.   D R A W I N G

June 18, 1974 w 5 u 1'z EI'AL 3,817,747

GARBURIZATION RBSISTAKT HIGH TEMPERATURE ALLOY Filed April 11, 1972 2 Shoots-Shoot 1 FIG.

June 18, 1974 J, w. SCHULTZ ETAL 3,817,747

CARBURIZATION RESISTANT HIGH TEMPERATURE ALLOY Filed April 11, 1972 2 Sheets-Shut I FIG United States Patent 3,817,747 CARBURIZATIO RESISTANT HIGH TEMPERATURE ALIDY Jay Ward Schultz, Sullern, and Russell Lawrence Mc- Carron, Warwick, N.Y., assignors to The International Nickel Company, Inc., New York, N.Y.

Filed Apr. 11, 1972, Ser. No. 242,980 Int. Cl. C22c 19/00 US. Cl. 75-471 13 Claims ABSTRACT OF THE DISCLOSURE Special heat resistant alloys containing nickel, chromium, iron, titanium, aluminum, silicon and carbon afford high temperature carburization, oxidation and sulfidation resistance, characteristics which render the alloys particularly suitable for use in contact with such media as hydrocarbons, high sulfur content feed stocks and the like.

The present invention relates to heat resistant alloys having excellent high temperature carburization, oxidation (including cyclic oxidation) and sulfidation resistance.

It is Well known that many industrial processes are now operating at temperatures of up to 1800 F. and above, and require equipment made of heat resistant alloys that will exhibit carburization, oxidation and sulfidation resistance at these temperatures. There is also a need for these alloys to have good welding characteristics so as to facilitate fabrication and maintenance of equipment.

One important industrial process that requires heat and corrosion resistant alloys is the production of ethylene by pyrolysis. This pyrolysis is carried out at high temperatures, usually in the range of 1600 F. to 1800" F in tubular reactors. The combination of high temperature and a hydrocarbon stream produces an environment in which a very serious carburization problem results. As in all industrial processes, long-time maintenance-free operation is an economic necessity and, in this case, the severe carburization problem results in substantially reducing the serviceable life of the equipment unless a suitable alloy is used. Alloys for ethylene pyrolysis furnace tubes must also have good resistance to stresses that can induce creep and rupture at the operating temperatures and good resistance to oxidation and sulfidation attack, especially when impure feedstocks are used.

In addition, advancements in technology, e.g., higher operating temperatures, processing of lower grade feedstocks and the like, will impose more stringent service requirements upon alloys for this application and increase the demands for strong, more corrosion resistant alloys.

We have now discovered a new alloy providing a new combination of characteristics especially suitable in equipment for the production of ethylene by pyrolysis and for other equipment subjected to similiar conditions of stress, temperature and corrosive attack.

It is an object of the invention to provide an alloy having a combination of characteristics including excellent resistance to carburization, oxidation, cyclic oxidation and sulfidation attack, good stress-rupture strength at high temperatures, and satisfactory weldability.

Another object of the invention is to provide heat and corrosion resistant alloy products and articles, including 3,817,747 Patented June 18, I974 products and articles for use in ethylene pyrolysis furnace tubes.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a reproduction of a photomicrograph taken at 250 diameters of Alloy 1 following heat treatment for 2 hours at 2300 F., water quenching, aging for one hour at 1800 F., water quenching and etching in aqueous 10% H FIG. 2 is a reproduction of a photomicrograph taken at 250 diameters of Alloy A following heat treatment for 2 hours at 2300 F., water quenching, againg for one hour at 1800 F., water quenching and etching in aqueous 10% H 80 Generally speaking, the present invention is directed to an alloy containing (by weight) from about 0.05% to about 0.15% carbon, from about 28% to about 35% chromium, from about 2.5% to about 6.0% aluminum, from about 10% to about 22% iron, from about 0.05% to about 0.8% titanium, up to about 1% silicon, the sum of the aluminum plus silicon being at least 3%, and the balance essentially nickel. As will be understood by those skilled in the art, the use of the expression "balance essentially" in referring to the nickel content of the alloys does not exclude the presence of other elements commonly present as incidental constituents and impurities.

In accordance with the invention, preferred alloys contemplated herein contain (by weight) from about 0.06% to 0.1% carbon, from about 30% to 34% chromium, from about 2.8% to 3.5% aluminum, from about 14% to 22% iron, from about 0.3% to 0.6% titanium, from about 0.4% to 0.6% silicon and the balance essentially nickel. Even more preferred alloys contain (by weight) from about 0.06% to 0.08% carbon, from about 31% to about 33% chromium, from about 2.9% to about 3.3% aluminum, from about 14% to about 17% iron, from about 0.3% to about 0.5% titanium, from about 0.4% to about 0.6% silicon and the balance essentially nickel.

The chromium content specified above herein is correlated according to the following mathematical relation (expressed in weight percent) to enable production of high creep-rupture strengths characteristic of the alloy: 28 Cr 39-(1.5A1+Si+Ti)-O.25(Fe-l6).

Alloys of the invention are characterized by a two-phase microstructure consisting of a gamma (face-centered cubic matrix with precipiated chromium-rich alpha prime (body-centered cubic) phase within the grains and at the grain boundaries. To obtain the proper microstructure, it is desirable to solution-treat the alloy for about two hours at about 2250 F. to 2350 F., water quench and then age for about one hour at about 1800" F,. followed by a water quench. The solution-treatment produces a larger grain size and the aging treatment precipitates the alpha prime phase, usually as fine, well-distributed bodies which may appear rod-like. The alpha prime phase markedly enhances the strength of the alloy, approximately doubling the room temperature tensile strength when the aging treatment is employed. If the alloy is to be used for applications up to about 1500 F., the aging treatment is required to precipitate the alpha prime phase. At applications above about 1500 F., the alpha prime phase will precipitate during exposure and increase the strength; thus, the aging treatment may be unnecessary.

3 In carrying the invention into practice, nickel is controlled in the amount of at least about 40% to provide a stable face-centered cubic matrix and the chromium is correlated to the contents of aluminum, silicon, titanium and iron according to the following mathematical relation (expressed in weight percent):

Chromium contributes carburization, oxidation and sulfidation resistance but must be controlled according to the foregoing relation to enable production of high creeprupture strengths characteristic of the alloy. Chromium levels substantially greater than those defined by the relation cause an excess of the alpha prime phase to form which does not dissolve at the solution temperature. The excess alpha prime phase inhibits grain growth in the alloy and results in a smaller grain size and reduced creeprupture strength. Chromium levels lower than about 28% result in decreased carburization, oxidation and sulfidation resistance and in an increased tendency for heatatfected-zone cracking during welding. Alloys which consistently manifest the best combination of creep strength, corrosion resistance and weldability contain chromium in accordance with the above relation and within the range of about 31% to 33%.

Aluminum enhances the carburization, oxidation and sulfidation resistance of the alloy. To maintain the desired carburization resistance it is essential that the aluminum content be maintained above about 2.5%. Aluminum at high levels above about 5% adversely affects the workability of the alloy. When workability is not a factor in producing the alloy, aluminum contents may be as high as about 6%. The best combination of properties is obtained when aluminum is maintained in a range of about 2.9% to 3.3%.

Silicon up to about 1% enhances carburization, oxidation and sulfidation resistance without significantly decreasing creep-rupture strength to achieve the desired combination of properties it is preferred to incorporate silicon in the range of about 0.4% to 0.6%. Silicon above about 1% adversely affects weldability.

Silicon and aluminum, within their respective ranges, may be varied to produce the desired carburization properties provided their sum be at least about 3% and most beneficially at least 3.3%.

Titanium is employed as a deoxidizer and denitrifier to enhance the hot workability of the alloy. A range of 0.3% to 0.6% titanium is preferred for this purpose. Other elements such as zirconium from about 0.05% to about 0.5% or 0.8%, boron up to about 0.1%, calcium up to about 0.05% and magnesium up to about 0.05%, could also be used singly or in combination for this purpose, in lieu of or together with the titanium. Zirconium and/or boron are particularly beneficial in conferring enhanced tensile ductility at temperatures on the order of about 1400 F.

At least about 0.05% carbon is necessary for high temperature strength, but should not substantially exceed about 0.15% in the interest of weldability. A range of 0.06% to 0.1% is preferred but more advantageously is from 0.06% to 0.08%.

Iron above about 22% causes weld cracking while percentages below about 10%, apart from other factors, unnecessarily increase cost. Iron in the range of 14% to 22% is preferred, but is more advantageously from about 14% to about 17%.

Commercial alloys embodying the present invention may also contain small amounts of other elements such as sulfur, phosphorus, manganese, copper, molybdenum and cobalt. Sulfur and phosphorus, for example, should be 4 maintained at levels consistent with good steel-making practice, levels less than about 0.030% and 0.045%, respectively.

For the purpose of giving those skilled in the art a better understanding of the invention, the following examples are given. Table I sets forth the compositions of Alloys 1 through 11 which are examples of alloys within the invention and Alloys A through D which are outside the invention. The series of alloys was vacuum melted in an electric induction furnace. The nickel, chromium and iron were charged into the furnace and heated to 2900 F. At this point, one-half of the aluminum and one-half of the titanium contents were added to the melts. The melts were held until all bubbling and agitation ceased and were then cooled to 2700 F., at which point the remainder of the aluminum and titanium was added along with the silicon and a high carbon-chrome addition alloy. The melts were then heated to 2750 F. and poured into 30 pound ingots. The ingots were soaked for two hours at 2200 F. and then rolled to two inch square bars which were then cut in half, heated to 2200 F. and then rolled to a /s-inch square bar. The alloys, except commercial Alloys C and D, were then subjected to either of two heat treatments: (A) Solution treat for two hours at 2300 F., water quench, age for one hour at 1800" F. and water q uench or (B) solution treat for two hours at 2200 F. and water quench. Alloy C was solution annealed for one hour at 1950 F. and air cooled. Alloy D was tested as cast.

The creep-rupture results set forth in Table II were obtained using standard testing procedures. The specimens were first creep-rupture tested followed by room temperature measurement of elongation and reduction of area.

The carburization tests set forth in Table III were run at 2012 F. in a flowing gas mixture of hydrogen containing 2 volume percent methane. The specimens were supported in ceramic fixtures and then inserted into a preheated tube furnace which was being flushed with argon. Following the argon flush, the hydrogen-methane gas mixture was introduced at a velocity of 0.5 cm./sec. over the specimens. At the end of each test period, the furnace was again flushed with argon and the specimens were removed to cool in air. The specimens were then lightly descaled to remove the oxide formed as the specimens were taken from the furnace, and the weight change of the specimens was measured. Descaling of all the test specimens was done with an 5.8. White precision abrasive cleaning unit using 50 micron alumina propelled by dry C0,. The penetration measurement was the depth of metal showing carbon penetration and was measured metallographically on a Leitz measuring microscope. All specimens were etched in modified Murakamis reagent prior to making the measurements. All the tests were run for a period of hours.

The sulfidation tests set forth in Table III were run in a flowing gas mixture of hydrogen containing 1.5 volume percent hydrogen sulfide at 1292 F. for 100 hours. The same testing procedure as that described above for the carburization tests was used.

The oxidation tests set forth in Table III were run in flowing air containing a controlled 5 volume percent water vapor at 2012 F. The air velocity over the specimens was 0.5 cm./sec. The tests were cyclic in that the specimens were removed from the furnace every 100 hours, cooled to room temperature, weighed and returned to the furnace. A total of 10 cycles (total test time of 1,000 hours) was employed. Test specimens were descaled at the end of the test following the procedures described above for the carburization tests.

TABLE I Calculated Cr Fe Ti Al 81 Cr l Calculated by difierence and may contain small amounts oi impurities and incidental elements not otherwise reported in this table.

3 Contains 0.93% manganese. Contains 0.68% manganese.

TABLE 11 Heat treatment A Heat treatment B Rupture Rupture time (hrs) time (hrs) 2,000 F./ El., R.A 2,000 F./ El., R.A., 2,500 p.s.i. percent percent 2,500 p.s.i. percent percent N 0'rE.-El.=elongation; R.A.=reduction in area.

Comparison of the properties of Alloys 1 and A in Table II clearly reflects the disastrous elfect on creeprupture strength caused by a chromium content substantially in excess of that defined by the above relation. The microstructures of the two alloys are shown, respectively, in FIGS. 1 and 2 of the drawing. As shown in FIG. 1, Alloy 1 is characterized by a large grain size and uniform distribution of the alpha prime phase which appears as fine, well distributed, darker-etching particles in the gamma matrix. In FIG. 2, on the other hand, Alloy A is seen to be characterized by a relatively smaller grain size and by a substantially greater proportion of the darker-etching alpha prime phase which itself occurs as considerably larger particles than was the case in FIG. 1. It is important to note that this reduction in creep-rupture strength (a factor of 10) is caused by a chromium content only 2.5% higher than the chromium content determined in accordance with the foregoing relationship.

It should be noted that Alloy 11 is, at best, a marginal alloy within the broadest limits of the invention. As can be seen from Table III, this alloy while strikingly surperior to the prior art Alloys C and D (also Alloy B) in terms of its ability to resist carburization, nonetheless behaved poorly, comparatively, speaking, relative to Alloys 1-10, the latter all having a total aluminum plus silicon content over 3.3%

In Table III two commercial alloys Alloys C and D, clearly demonstrate the relatively poor corrosion properties of materials currently in use for ethylene pyrolysis furnace tubes under the severe testing conditions employed.

The weldability of the alloy is demonstrated by tests conducted on Alloys 1 through 3. Plates of Alloy 1 Were surface ground, gas tungsten-arc welded for bead-on-plate and thermal shock tests and visually examined at 10X for evidence of weld and heat-alfected zone defects. The bcad-on-plate test was conducted at 11 volts, 250 amperes, with one pass at a travel speed of 16 inches per minute (i'p.m.) and an argon flow of 25 cubic feet per hour (cf./h.). Each test was conducted using a non-consumable tungsten electrode and no filler material. In the bead-on-plate test there was no evidence of defects and in the thermal shock test there was only crater cracking and one weld crack. Crater cracking, as will be appreciated by those skilled in the art is, in part, a function of the skill exercised in making the weld and, even it they do occur during the welding operation, are normally melted out.

One-half inch thick plate (60 V bevel) of Alloys 2 and 3 were manually gas tungsten-arc butt welded using the parent metal composition as filler /s" diameter). The weld was conducted at 16 volts, 230 amperes, with 9 passes at a travel speed of approximately 3 i.p.rn. and an argon flow of 25 cf./h. These joints were radiographically inspected and showed no indication of weld cracking. They were also cut into one-half inch wide transverse slices, polished and etched with Lepitos reagent. Examination at 10X for weld and heat-affected zone defects showed satisfactory welds with relatively few cracks.

The alloys of the invention are especially useful in applications involving the processing of hydrocarbons and sulfidizing and oxidizing materials at high temperatures.

They can be employed in many other applications, including high temperature application, where resistance to corrosion and good creep and rupture properties are required. Alloys of the invention may be either wrought or cast and exemplary articles include ethylene pyrolysis furnace tubes, piping, valves, vessels and other equipment used in industrial chemical plants. For cast alloys the silicon can beup to 1.5 or 2%.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A nickel-base heat resistant alloy affording high temperature carburization, oxidation, cyclic oxidation and sulfidation resistance consisting essentially of about 0.05% to about 0.15% carbon, from about 28% to about 35% chromium, in which the chromium content is correlated according to the formula from which about 2.5% to about 6% aluminum, up to 2% silicon, with the sum of the aluminum plus silicon being at least 3% and the silicon not exceeding about 1% in respect of wrought weldable alloys, from about 10% to about 22% iron, from about 0.05 to about 0.8% titanium, and the balance essentially nickel.

2. As a new article of manufacture, a component fabricated from the alloy set forth in claim 1 for use as a furnace tube in the pyrolysis of ethylene.

3. The article set forth in claim 2 in which the silicon content of the alloy does not exceed 1% 4. A nickel-base alloy in accordance with claim 1 wherein the titanium is wholly or partly replaced by at least one element selected from the group consisting of from about 0.05 to about 0.8% zirconium, up to about 0.1% boron, up to about 0.05% calcium and up to about 0.05% magnesium.

5. A nickel-base alloy in accordance with claim 1 containing about 0.06% to about 0.1% carbon, about 30% to about 34% chromium, about 2.8% to about 3.5% aluminium, about 14% to about 22% iron, about 0.3% to about 0.6% titanium, and about 0.4% to about 0.6% silicon, the aluminum plus silicon being at least about 3.3%.

6. As a new article of manufacture, a component fabricated from the alloy set forth in claim for use as a furnace tube in the pyrolysis of ethylene.

7. An alloy in accordance with claim 1 in which the silicon content does not exceed 1.5%

8. A nickel-base heat resistant alloy affording high temperature carburization, oxidation, cyclic oxidation and sulfidation resistance consisting essentially of about 0.06% to about 0.08% carbon, from about 31% to about 33% chromium, from about 2.9% to about 3.3% aluminum, from about 14% to about 17% iron, from 0.3% to about 0.5% titanium, from about 0.4% to about 0.6% silicon and the balance essentially nickel.

9. As a new article of manufacture, a component fabricated from the alloy set forth in claim 8 for use as a furnace tube in the pyrolysis of ethylene.

10. A nickel-base heat resistant alloy affording high temperature carburization, oxidation, cyclic oxidation and sulfidation resistance consisting essentially of about 0.05% to about 0.15% carbon, from about 28% to about 35% chromium, in which the chromium content is correlated according to the formula 28 Cr 39 LSAH-SH-Ti) 0.25(Fe16),

from about 2.5% to about 6% aluminum, up to 2% silicon, with the sum of the aluminum plus silicon being at least 3% and the silicon not exceeding about 1% in respect of wrought weldable alloys, from about 10% to about 22% iron, from about 0.05% to aobut 0.8% titanium, and the balance essentially nickel, said alloy being further charatcerized by a two-phase microstructure consisting of a gamma matrix with precipitated chromiumrich alpha prime phase within the grains and at the grain boundaries.

11. A nickel-base alloy in accordance with claim 10 containing about 0.06% to about 0.1% carbon, about 30% to about 34% chromium, about 2.8% to about 3.5% aluminum, about 14% to about 22% iron, about 0.3% to about 0.6% titanium, and about 0.4% to about 0.6% silicon, the aluminum plus silicon being at least about 3.3%.

12. The method for producing in nickel-base alloys containing about 0.05% to about 0.15% carbon, from about 28% to about 35% chromium, in which the chromium content is correlated according to the formula 28 Cr 39 (1.5Al-I-Si-i-Ti) 0.25(Fe-16),

from about 2.5% to about 6% aluminum, up to 2% silicon, with the sum of the aluminum plus silicon being at least 3% and the silicon not exceeding about 1% in respect of wrought weldable alloys, from about 10% to about 22% iron, from about 0.05% to about 0.8% titanium, and the balance essentially nickel, a microstructure consisting of a gamma matrix with precipitated chromium-rich alpha prime phase within the grains and at the grain boundaries which comprises heating said alloy at a temperature in the range of about 2250 F. to about 2350 F. for about 2 hours and quenching.

13. A method in accordance with claim 12 further comprising reheating said alloy for about one hour at a temperature of about 1800' F. and quenching.

References Cited UNITED STATES PATENTS 3,690,873 9/1972 Fontaine 171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

